Patent GB-1186783-A (GB)
Title: Atmospheric Condition Indicating Devices
IPC: G01N25/66
Filed: 1967-04-20
Abstract: 1,186,783. Detecting moisture electrically. HOLLEY CARBURETOR CO. April 20, 1967 [June 21, 1966], No. 18214/67. Heading G1N. In a detector for the approach of icing conditions at a surface 13, Fig. 2, means such as a relay 48 is operated by an and-gate 34 when the following conditions are satisfied: (1) the surface temperature T s , measured at 12, is less than a reference temperature such as freezing point T f  at 26, these being compared at 20, to give a signal T to the gate 34, and (2) the relative humidity H of the atmosphere measured at 40 bears a predetermined relationship to another humidity signal such as a reference value derived from the difference #T 1  between T s  and the ambient temperature Ta measured at 30, the two temperatures being compared at 22. The signals H and #T 1  are fed to a circuit 38 which determines whether the two signals define a point lying above or below the curve (D) of the graph Fig. 1, (not shown) relating the two quantities at an average saturation temperature, a point above the curve (D) indicating that condensation will occur and a point below the curve indicating that condensation will not occur. A corresponding signal C is sent to the gate 34. The curve (D) is the average of a number of curves drawn for different saturation temperatures in the practical range of interest, namely 0 F. to 32 F., and as these curves (A, B, C) are close together, their average (D) is a sufficiently accurate approximation. An alternative, less accurate, approximation is to assume that a given temperature difference T 2  (line F), must be exceeded and that a given humidity (line E) must also be exceeded for condensation to occur. The detector of Fig. 1 is modified so that relay 48 operates when the following conditions are satisfied Fig. 6 (not shown): (1) surface temperature, T s , is less than freezing, as before, (2) the relative humidity (H) is greater than a reference value (set at 72) with which it is compared (at 74) and (3) the ambient temperature Ta is greater than the surface temperature T s  by a given amount, which amount is pre-set in a comparator (70) receiving the two signals. In a further modification Fig. 7 (not shown) the ambient temperature Ta is compared with a fixed reference (82), the assumption being that a constant temperature differential T s -Ta exists, and that Ta must be higher than a certain value (the reference value) to produce condensation. To avoid failure of the device to detect ice resulting from precipitation, such as rain, as opposed to condensation, a heated moisture detector 114 and an unheated one 112 are used, a significant difference in resistance being detected between them when ice forms on the unheated one while the heated one remains wet. The signal indicating ice approach is used to operate heaters or warning signals on, for example, road surfaces, or gas turbine inlets. The condensation detector Fig. 4 (not shown), comprises a cylindrical outer electrode (124) an inverted cup-shaped inner electrode (132), insulation between them (130, 134), and a thermistor (144) sensing the temperature; and in the heated transducer a heater resistance chain (148, Fig. 5, not shown). The detector is connected Fig. 3 (not shown) to control the pulse width of the output of a pulse train generator (64).